The present invention relates to a plate heat exchanger for evaporating a fluid, comprising a package of abutting rectangular and essentially vertically arranged heat transfer plates, delimiting flow spaces between themselves and provided with corrugation patterns of ridges and grooves, said ridges intersectingly abutting each other in at least a part of each flow space and forming a number of supporting points between adjacent heat transfer plates, wherein each alternate flow space forms an evaporating passage, which evaporating passage has an inlet for fluid at its lower portion and an outlet for fluid and generated vapour at its upper portion near one of the vertical sides of the heat transfer plates, and the remaining flow spaces form passages for a heating fluid, which passages have inlets at their upper portions near the other vertical sides of the heat transfer plates and outlets at their lower portions.
In a known plate heat exchanger of this kind, described in DE-3721132, the main part of the heat transfer portion of each heat transfer plate has one and the same kind of corrugation pattern over its entire surface. This is ineffective with respect of the heat transfer capacity of the plate heat exchanger. In the previously known plate heat exchanger an outlet duct for fluid and generated vapour extends further through the package of heat transfer plates, which outlet duct is formed of aligned openings of the heat transfer plates. The openings are made as great as possible to minimize the flow resistance in the outlet duct for the produced vapour. In practice a large part of the upper portion of each heat transfer plate has been used for such opening. As an inlet duct, intended for the heating fluid, must also extend through the upper part of the package of heat transfer plates, it has not been possible to use the entire width of the heat transfer plates only for the outlet duct. This has resulted in flow paths of different length being formed in each evaporating passage between its inlet and its outlet for different parts of supplied fluid and vapour generated therefrom.
Owing to the known heat transfer plates having one kind of corrugation pattern over their heat transfer portions and thereby causing equal flow resistance per unit of length of each flow path for fluid and generated vapour in each evaporating passage, the total flow resistance will be largest along the longest flow path. Consequently, the smallest amount of fluid and vapour passes this path. This will lead to not all of the fluid being treated to the same heat treatment and the risk of drying out exists along the longest flow path, above all, near the inlet of the heating fluid.
EP 0 477 346 B1 describes an improved heat exchanger plate where the heat exchanger plates are divided in different zones, where the zones are provided with different corrugation patterns. In this way, the flow resistance through a fluid channel is optimized.
EP 0 458 555 B1 describes a further improved heat exchanger plate in which a lower heat transfer area is horizontally divided in different portions and upper lower heat transfer area which is vertically divided. The smallest angle for any of the portions of the lower heat transfer area has substantially the same size as any of the angles in upper heat transfer area. Thereby an even and improved flow distribution is achieved in the fluid channel from the inlet and onwards.
Even though these known heat exchanger plates show a favourable efficiency and have proved to be a commercial success, there is still room for improvements.